These books, with of a total of 40 chapters, are a comprehensive and complete introductory text on the synthesis, characterization, and applications of nanomaterials. They are aimed at graduate students and researchers whose background is chemistry, physics, materials science, chemical engineering, electrical engineering, and biomedical science.
The first part emphasizes the chemical and physical approaches used for synthesis of nanomaterials. The second part emphasizes the techniques used for characterizing the structure and properties of nanomaterials, aiming at describing the physical mechanism, data interpretation, and detailed applications of the techniques. The final part focuses on systems of different nanostructural materials with novel properties and applications.

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Volume I: Synthesis
Content
1.1 INTRODUCTION...................................................................................................................... 1
1.2 FORMATION MECHANISMS OF MICELLES AND MICROEMULSIONS ........................................ 3
1.2.1 Simple Geometric Factors ............................................................................................ 3
1.2.2 The Critical Micelle Concentration (CMC) for Surfactants......................................... 5
1.2.3 Solubilization and Formation of Microemulsions ........................................................ 6
1.3 SYNTHESIS OF NANOPARTICLES FROM W/O MICROEMULSIONS (REVERSED MICELLES) ...... 9
1.3.1 Preparation of Nanoparticles of Metals ..................................................................... 10
1.3.2 Preparation of Nanoparticles of Metal Sulfides ......................................................... 11
1.3.3 Preparation of Nanoparticles of Metal Salts.............................................................. 12
1.3.4 Preparation of Nanoparticles of Metal Oxides........................................................... 12
1.3.5 Preparation of Nanoparticles of Other Compounds................................................... 13
1.3.6 Synthesis of Nanowires Using Reversed Micelles ...................................................... 13
1.3.7 Synthesis of Composite Nanoparticles Using Reversed Micelles ............................... 14
1.4 SYNTHESIS OF ORGANIC NANOPARTICLES FROM O/W MICROEMULSIONS .......................... 14
1.4.1 Introduction ................................................................................................................ 14
1.4.2 Synthesis of Styrene Latex Nanoparticles from O/W Microemulsions........................ 15
1.4.3 Synthesis of Methylmethacrylate Nanoparticles from O/W Microemulsions.............. 16
1.5 APPLICATIONS ..................................................................................................................... 16
1.6 PROSPECTS .......................................................................................................................... 17
References ........................................................................................................................... 17
2.1 INTRODUCTION.................................................................................................................... 23
2.2 METAL NANOPARTICLES ..................................................................................................... 25
2.2.1 Background ................................................................................................................ 25
2.2.2 Precious Metal Nanoparticles .................................................................................... 26
2.2.3 Transition Metal Nanoparticles.................................................................................. 27
2.3 OXIDE NANOPARTICLES ...................................................................................................... 30
2.3.1 General Background of Nano-Oxides ........................................................................ 30
2.3.2 Ceramic Oxide Nanoparticles .................................................................................... 31
2.3.3 Specific Ceramic—SiC ............................................................................................... 33
2.3.4 Functional Oxide Nanoparticles................................................................................. 33
2.4 COMPOUND SEMICONDUCTOR NANOPARTICLES ................................................................. 35
2.4.1 Background ................................................................................................................ 35
2.4.2 III-V Semiconductor Nanoparticles............................................................................ 35
2.4.3 II-VI Semiconductor Nanoparticles............................................................................ 36
2.4.4 Other Typical Semiconductor Nanoparticles ............................................................. 38
2.4.5 Conclusions ................................................................................................................ 38
2.5 SUPERCONDUCTOR NANOMATERIALS ................................................................................. 39
2.5.1 Background ................................................................................................................ 39
2.5.2 YBCO Cuprates .......................................................................................................... 39
2.5.3 Bi-Series Cuprates...................................................................................................... 40
2.5.4 Tl-Series Cuprates ...................................................................................................... 40
2.6 ELEMENT NANOSTRUCTURES .............................................................................................. 40
2.6.1 Background ................................................................................................................ 40
2.6.2 Carbon Nanosystems .................................................................................................. 41
2.6.3 IV Semiconductor Nanoclusters ................................................................................. 42
2.7 INTRAZEOLITE TOPOTAXY IN MESOPOROUS MATERIALS .................................................... 43
2
2.8 CONCLUSIONS ..................................................................................................................... 44
References ........................................................................................................................... 44
3.1 INTRODUCTION.................................................................................................................... 47
3.2 FORCED HYDROLYSIS AND CONTROLLED RELEASE OF ANIONS .......................................... 48
3.2.1 Forced Hydrolysis ...................................................................................................... 48
3.2.2 Precipitation by Controlled Release of Anions........................................................... 49
3.2.3 Nucleation and Growth .............................................................................................. 51
3.2.4 Factors Controlling Particle Sizes ............................................................................. 55
3.3 CHEMICAL CO-PRECIPITATION............................................................................................ 59
References ........................................................................................................................... 62
4.1 INTRODUCTION.................................................................................................................... 63
4.2 PRINCIPLES OF THE SYNTHESIS TECHNIQUE ........................................................................ 64
4.3 EXPERIMENTAL APPROACH ................................................................................................. 65
4.3.1 Silica Sol-Gel Processing ........................................................................................... 65
4.3.2 Metal Alkoxide Method............................................................................................... 70
4.3.3 Pechini Processing ..................................................................................................... 75
4.3.4 Sol-gel Thin Film Processing ..................................................................................... 78
4.4 EXAMPLES OF THE SYNTHESIS PROCESS ............................................................................. 79
4.4.1 Organic/Inorganic Hybrid Network Materials Based on Silica sol-gel Approach—
PTMO TMOS nanocomposites (Huang, et al., 1987).......................................................... 80
4.4.2 Composited Oxide Powders—MgAl2O4 spinel from a heterometallic alkoxide
(Varnier, et al., 1994) .......................................................................................................... 82
4.4.3 Nanocrystalline Thin Films of Composited Oxides—Co and RE(rare earth)-doped Co
ferrite nanocrystalline films (Cheng, et al., 1998; Yan, et al., 1998; Cheng, et al., 1999).. 83
4.4.4 Glass/Non-Oxide Nanocomposites by sol-gel Technique—LaF3 microcrystals in solgel silica .............................................................................................................................. 86
4.5 CURRENT STATUS OF THE TECHNIQUE, LIMITATIONS AND PROSPECTS ............................... 87
References ........................................................................................................................... 87
5.1 INTRODUCTION.................................................................................................................... 91
5.2 PRINCIPLES OF CHEMICAL VAPOR DEPOSITION ................................................................... 92
5.3 EXPERIMENTAL APPROACH ................................................................................................. 95
5.3.1 Chemical Vapor Deposition (CVD)............................................................................ 96
5.3.2 Chemical Vapor Condensation (CVC) ....................................................................... 96
5.3.3 Particle–Precipitation–Aided Chemical Vapor Deposition ....................................... 97
5.3.4 Catalytic Chemical Vapor Deposition........................................................................ 99
5.4 EXAMPLES OF NANOSTRUCTURED MATERIALS ................................................................. 100
5.4.1 Semiconductor Quantum Dots.................................................................................. 100
5.4.2 Ceramic Nanostructured Materials.......................................................................... 104
5.4.3 Carbon Nanotubes.................................................................................................... 112
5.4.4 Diamond ................................................................................................................... 121
5.5 SUMMARY ......................................................................................................................... 126
References ......................................................................................................................... 127
6.1 INTRODUCTION.................................................................................................................. 131
6.2 PRINCIPALS OF AEROSOL SYNTHESIS/THEORY.................................................................. 134
6.2.1 Early Work ............................................................................................................... 135
6.2.2 Homogeneous Nucleation......................................................................................... 137
6.2.3 Collision-Coalescence Growth................................................................................. 141
6.2.4 Forced Flow Production .......................................................................................... 153
6.3 EXPERIMENTAL ................................................................................................................. 159
6.3.1 Inert Gas Condensation Methods ............................................................................. 160
6.3.2 Arc (Spark) Evaporation Sources............................................................................. 166
6.3.3 Gas-Phase Reaction in a Free Jet ............................................................................ 170
3
6.3.4 Laser Ablation and Laser Driven Chemical Reaction Sources ................................ 172
6.3.5 Sputtering ................................................................................................................. 175
6.4 CONCLUSIONS ................................................................................................................... 176
References ......................................................................................................................... 177
7.1 INTRODUCTION.................................................................................................................. 180
7.2 SPUTTERING ...................................................................................................................... 181
7.2.1 Principle of Sputtering.............................................................................................. 181
7.2.2 Sputtering Systems.................................................................................................... 181
7.2.3 Examples of Multilayer Structures Prepared by Sputtering ..................................... 184
7.2.4 Current Status of Sputtering ..................................................................................... 188
7.3 PULSED LASER DEPOSITION .............................................................................................. 189
7.3.1 Principle of Pulsed Laser Deposition....................................................................... 189
7.3.2 Deposition of Nano-Scale Metal Oxide Thin Films.................................................. 190
7.3.3 Examples of Multilayer Structures Prepared by Pulsed Laser Deposition .............. 194
7.3.4 Current Status of Pulsed Laser Deposition .............................................................. 197
References ......................................................................................................................... 197
8.1 INTRODUCTION.................................................................................................................. 199
8.2 PRINCIPLES OF LASER ABLATION ...................................................................................... 204
8.2.1 Fundamental Process ............................................................................................... 204
8.2.2 Theoretical Model .................................................................................................... 205
8.3 PROCESSING EXPERIMENTS ............................................................................................... 210
8.3.1 Process Chamber...................................................................................................... 210
8.3.2 Processing Procedures ............................................................................................. 212
8.3.3 Laser Absorption Spectroscopy ................................................................................ 213
8.3.4 Process Variables..................................................................................................... 218
8.4 MICROSTRUCTURE ............................................................................................................ 221
8.4.1 NbAl3 Nanocrystalline Powders ............................................................................... 221
8.4.2 NbAl3/Al Multilayer Thin Film ................................................................................. 224
8.5 CONCLUSIONS ................................................................................................................... 226
References ......................................................................................................................... 226
9.1 INTRODUCTION.................................................................................................................. 229
9.2 PHYSICAL VAPOR DEPOSITION: EVAPORATION AND SPUTTERING .................................... 229
9.2.1 Deposition: Film Nucleation and Growth ................................................................ 229
9.2.2 Evaporation .............................................................................................................. 231
9.2.3 Sputtering ................................................................................................................. 232
9.2.4 Examples .................................................................................................................. 233
9.3 THERMAL SPRAYING ......................................................................................................... 242
9.4 ELECTRODEPOSITION AND ELECTROLESS DEPOSITION ...................................................... 245
9.5 SUMMARY ......................................................................................................................... 252
References ......................................................................................................................... 252
10.1 INTRODUCTION................................................................................................................ 258
10.2 NANOLITHOGRAPHY TECHNIQUES .................................................................................. 259
10.2.1 Electron Beam Lithography (EBL)......................................................................... 259
10.2.2 X-ray Lithography (XRL)........................................................................................ 266
10.2.3 Extreme Ultraviolet Lithography (EUVL) .............................................................. 271
10.3 EXAMPLES OF THE ARTIFICIAL PATTERNED NANOSTRUCTURES ..................................... 273
10.4 SUMMARY AND PROSPECTS ............................................................................................. 277
References ......................................................................................................................... 278
11.1 INTRODUCTION................................................................................................................ 280
11.2 ION IMPLANTATION FACILITY ......................................................................................... 281
11.3 ION-SOLID INTERACTIONS ............................................................................................... 283
11.3.1 Ion Stopping Mechanisms....................................................................................... 283
4
11.3.2 Nuclear Stopping .................................................................................................... 285
11.3.3 Electronic Stopping ................................................................................................ 293
11.3.4 Ion Ranges.............................................................................................................. 295
11.3.5 Channeling ............................................................................................................. 297
11.3.6 Sputtering ............................................................................................................... 299
11.3.7 Radiation Damage.................................................................................................. 300
11.4 ALLOYING, AMORPHIZATION AND PHASE TRANSFORMATION......................................... 304
11.5 NANOCRYSTALLINE PHASES CREATED BY ION IMPLANTATION ...................................... 307
11.5.1 Ion Implantation and Nucleation............................................................................ 312
11.5.2 Influence of the Matrix Structure on the Nanocrystal Structure and Orientation .. 313
11.5.3 Nanocrystal Size Control........................................................................................ 316
11.6 ION BEAM MIXING AND SPUTTER DEPOSITION ............................................................... 319
References ......................................................................................................................... 320
APPENDIX ............................................................................................................................... 325
5
1.1 Introduction
Nanoparticles play a vital role in high performance materials in high technology
industries. The studies of nanoparticles started in the early 1980's and have now become
one of the hottest worldwide research fields (Pui and Chen, 1997).
There are four main processing approaches for the preparation of nanoparticles by
chemical method (Riman, 1993): (1) chemistry in liquid phase including direct strike
(Murata, et al., 1976), nonsolvent addition (Mulder, 1970), solvent removal (Cheng, et
al., 1986), gel drying (sol-gel) (Perthuis, And Colomban, 1984) and precipitation from
homogeneous solution (Gordon, et al., 1959); (2) chemistry between heterogeneous
phase including hydrothermal synthesis (Adair, et al., 1987), molten salt synthesis
(Arendt, et al., 1979), pyrolysis (Wada, et al., 1987) and spark erosion (Berkowitz, et al.,
1987); (3) chemistry in a droplet including emulsions (Woodhead, et al., 1980), micelles
(Gobe, et al., 1983) or microemulsions (Kandori, et al., 1988) and aerosols (Balboa, et
al., 1987); (4) chemistry in the vapor phase including heating method (Mazdiyasni, et al.,
1965), vapor precursors (Iwama, et al., 1982), liquid precursors (Kagawa, et al., 1983)
and solid precursors (Watanabe, et al., 1986). The most attractive methods are those
which synthesize in the liquid medium, including methods of precipitation, reduction,
dehydration, solvent evaporation, reversed micelle technology and microemulsion
polymerization, etc. In this chapter, we will focus on the nanoparticles made from both
W/O microemulsion (reversed micelles) and O/W microemulsion procedures.
Hence it is necessary to introduce the definition of micelles and microemulsions before
dealing with the principles and practices of forming nanoparticles from micelles and
microemulsions. Micelles are aggregates of surfactants in a liquid medium which are
formed when the surfactant concentration exceeds the critical micelle concentration
(CMC) (McBain and Salmon, 1920). It must be mentioned that this definition is only for
normal micelles; for the case of reversed micelles it is not necessary to have a CMC. In
the normal micelle the surfactant is orientated in such a way that the hydrophobic
hydrocarbon chains are towards the interior of the micelle, leaving the hydrophilic
groups in contact with the aqueous medium. Above the CMC, the physical state of the
surfactant molecules dissolved in water changes dramatically, and additional surfactant
exists as aggregates or micelles. Thus, the bulk properties of the surfactant, such as
osmotic pressure, turbidity, solubilization, surface tension, conductivity and selfdiffusion, change around the critical micelle concentration (Fig. 1.1).
Figure 1.1 Changes in concentration dependence of a wide range of physicochemical quantities around the critical micelle concentration (After Lindman,
1980).
1
If the micelles are formed in non-aqueous medium, the aggregates are called reversed
micelles, as in this case the hydrophilic head groups are now towards the core of the
micelle while leaving the hydrophobic groups outside of the micelles. The driving force
for formation of reversed micells is the dipole-dipole interactions of the surfactant. The
number of aggregates is usually small and not sensitive to the surfactant concentration
and thus there is no obvious CMC (Zhao, 1991; Gutmann and Kertes, 1973; Kertes and
Gutmann, 1976). In both cases (micelles and reversed micelles), only a small amount of
solubilized hydrophobic (usually oil) or hydrophilic (usually water) material exists in the
micelles (Fig. 1.2). However, the solubilization can be enhanced if the concentration of
surfactant is increased further. As the inside pool of water or oil is enlarged or swollen,
the droplet size increases up to a dimension much larger than the monolayer thickness of
the surfactants. In this case, we call them microemulsions or swollen micelles. What we
now describe as the preparation of nanoparticles from the reversed micelles may be
better described as preparation from swollen reversed micelles or water-in-oil
microemulsions.
Figure 1.2 The structure of micelles and microemulsions (O/W and W/O) (After
Overbeek et al., 1983).
As the surfactant concentration increases further, micelles can be deformed and can
change their shapes to rodlike micelles, hexagonal micelles and lamellar micelles or
2
liquid crystals (Fig. 1.3). It is these changes that make it possible to prepare different
shapes of nanoparticles from micelle synthesis microreactors.
1.2 Formation Mechanisms of Micelles and Microemulsions
1.2.1 Simple Geometric Factors
The structures of micelles can be simply determined by the geometric factors of the
surfactant at the interface, including head group area a0, the alkyl chain volume v and the
maximum length lc (to which the alkyl chain can extend). According to Israelachvili
(Israelachvili, et al., 1976), the packing considerations govern the geometry of
aggregation into micelles, vesicles and liposomes.
Figure 1.3 A schematic phase diagram of surfactant-oil-water systems showing
a variety of self-assembled structures (After Liu, J., et al., 1996).
These obey the following rules:
1.
2.
3.
4.
Spherical micelles require v/a0lc < 1/3,
Non-spherical micelles require 1/3 < v/a0lc < 1/2,
Vesicles or bilayers require 1/2 < v/a0lc < 1, and
Inverted micelles require 1 < v/a0lc.
In each case, the limits for the packing parameter v/a0lc can be evaluated from simple
geometry (Fig. 1.4) (Israelachvili, 1985). However, the change of environment will
affect these parameters, and thus dictate the molecular packing at the interface.
3
Figure 1.4 The relationship between aggregate type and geometry on the
packing requirements of surfactant head group and chains (Israelachvili, 1985).
4
1.2.1.1 Spherical Micelles
Spherical micelles are usually formed by anionic surfactants with or without cosurfactants. For an O/W micelle, this can be done by adjusting the repulsion (double
layers) between adjacent head groups, resulting in large values for a0. In this case, the
micelle radius is approximately equal to the maximum stretched out length of the
surfactant molecule and therefore the aggregates are very small. Bellare et al. (1988),
using small-angle neutron scattering (SANS), have successfully visualized a spherical
micelle of radius (3.0 ± 0.3) nm for a cryo-TEM image of a 10 mmol • dm-3 solution of
ditetradecyl-dimethyl-ammonium acetate.
1.2.1.2 Cylindrical Micelles
It is a quite common phenomenon that micelles grow as the preferred surface curvature
decreases. Any change that reduces the effective head group area will lead to the growth
of micelles. There are basically three ways to form cylindrical micelles: (1) addition of a
co-surfactant with a very compact head group, i.e. n-alkanol for which the–OH group is
small in comparison with a charged sulfate group, (2) changing the counterion, i.e.,
changing Na+ to Mg2+ will significantly reduce the electric double layer thickness, and
hence reduce the effective volume (size) of the head groups, (3) changing the
hydrophilicity of non-ionic head groups by electrolyte addition or temperature change;
i.e., for micelles formed by surfactants with poly(oxyethylene)(PEO) head groups, the
head groups are sensitive to changes of solvency (Tadros, 1987).
1.2.2 The Critical Micelle Concentration (CMC) for Surfactants
The CMC of a surfactant system depends on the minimum value of the interaction free
energy per molecule µN0. The minimum arises from the hydrophilicity of the head group,
tending to increase the area per molecule, while the hydrophobicity of the alkyl tail
tends to cause a decrease due to the hydrophobic bonding. From this concept, one is able
to predict how various structural features of surfactant molecules will affect their CMC
values.
Table 1.1 Typical CMC values for ionic surfactants at 25 °C
5
Surfactant
CMC/mmol • dm-3
C12H25SO4Na
8.1
C12H25SO4Li
8.9
C12H25SO3Na
10
C12H25CO3K
12.5
C12H25NH3Cl
14.7
C12H25NC2H5Cl
15
16
C12H25N(CH3)3Br
C12H25N(CH3)3Cl
17
For these ionic surfactants, there is little difference between anionic and cationic head
groups, since both have comparatively high CMC values, provided that the counterion is
monovalent. Usually, the CMC values for these systems are 1–20 mmol • dm-3 (Table
1.1). However, to change the counterion to a multivalent one tends to decrease the CMC
considerably.
For non-ionic surfactants, such as CxEy type, where x is the carbon number in the range
of 8–18, and y is the ethylene oxide group in the range of 3–20, the CMC value is
extremely low, i.e., 0.04–3 mmol • dm-3, depending on the structure of the molecules
(Table 1.2).
Table 1.2 Typical CMC values for non-ionic surfactants at
25 °C
Surfactant
CMC/mmol · dm-3
C12H25(OCH2CH2)4OH
0.046
C12H25(OCH2CH2)6OH
0.087
C12H25(OCH2CH2)8OH
0.109
C12H25(CH3)NO
2.1
1.2.3 Solubilization and Formation of Microemulsions
1.2.3.1 Solubilization
The term solubilization in this chapter refers to the dissolution of hydrophobic
(hydrophilic) materials into water (or oil) to an extent greatly exceeding their normal
solubilities in water (oil). The interior of a micelle provides a hydrophobic (hydrophilic)
environment in which non-polar (or polar) compounds can be accommodated. As a
result, the solubility of hydrophobic (or hydrophilic) material increases dramatically
with increasing surfactant concentration when it reaches the CMC as shown in Fig. 1.1.
The solubility behavior of surfactants is anomalous as the temperature is increased to a
value at which there is a sudden increase in solubility and the material then becomes
very highly soluble (Krafft, 1899). This is illustrated in Fig. 1.5.
6
Figure 1.5 Schematic representation of solubility versus temperature showing
location of the Krafft point (After Shinoda, 1974).
The process of solubilization has many applications in industrial preparations, for
example, in solubilization of insoluble drugs for intravenous injection. The process of
solubilization by micellar systems is also important in detergency, whereby fats and oils
are removed by incorporation into the hydrocarbon core of the micelle. There are four
general possible ways for the incorporation of the solubilization: (1) in the hydrocarbon
core of the micelle; (2) orientation in the micelle which could be deep or shallow; (3) in
the hydrophilic portion of the surfactant (e.g., ethylene oxide of non-ionic surfactants);
and (4) adsorption on the surface of the micelle (Fig. 1.6).
Figure 1.6 Schematic representation of four ways of solubilization of micelles.
7
1.2.3.2 Microemulsions
The microemulsion systems were first reported by Hoar and Schulman (1943), who
described transparent or translucent systems, formed spontaneously when oil and water
were mixed with a relatively large amount of an ionic surfactant combined with a
cosurfactant, e.g., a medium size alcohol. Later, in 1959, Schulman and co-workers
(Schulman, et al., 1959) introduced the concept of microemulsions as transparent or
translucent systems with a spherical or cylindrical size range of 8–100 nm. This is the
right size for preparing spherical and rod-like nanometer particles.
The solubilization theories of microemulsions have been proposed by Shinoda (Shinoda,
1974), who considered microemulsions as solubilized systems extended from the threecomponent phase diagrams of water-surfactant and co-surfactant (Fig. 1.7). It is clear
that in the phase diagrams there are two isotropic regions: one in the top corner, the so
called L2 phase or inverse micelles, and one in the left corner, i.e., L1 phase or normal
micelles. The L2 phase is capable of dissolving a large amount of water, thereby forming
a W/O microemulsion. Similarly, the L1 phase can solubilize oil to form an O/W
microemulsion. Thus, O/W microemulsions can be considered as an extension of the L2
phase, whereas W/O microemulsions can be considered as an extension of the L1 phase.
Figure 1.7 Schematic representation of a tree-component phase diagram for
water-surfactant and cosurfactant (After Overbeek et al., 1983).
The advantages of microemulsions in many industrial processes are distinct: from their
spontaneous formation, thermodynamic stability to lack of aging. Applications are based
on the low interfacial tension (as in tertiary oil recovery), the possibility of preparing
both hydrophilic and hydrophobic nearly homogeneous nanoparticles, the small droplet
size produced and their isodisperse nature.
8
1.3 Synthesis of Nanoparticles from W/O Microemulsions (Reversed Micelles)
O/W microemulsions (reversed micelles) can be formed by ionic surfactants with double
long alkyl chains alone, such as, AOT (Aerosol OT) by or a mixture of ionic and
nonionic surfactants with a short oxyethylene chain dissolved in organic solvents.
Reversed micelles are usually thermodynamically stable mixtures of four components:
surfactant, co-surfactant, organic solvent and water. AOT, SDS (sodium dodecyl sulfate),
CTAB (cetyltrimethy lammonium bromide) and Triton-X are the usual surfactants. Cosurfactants are often aliphatic alcohols with a chain length of C6–C8. Organic solvents
used for reversed micelle formation are usually alkane or cycloalkane with 6 to 8
carbons.
Reversed micelles can solubilize relatively large amounts of water. It is this water pool
that makes the reversed micelles particularly favorable for the synthesis of nanoparticles
because the water pool is in the range of nanometer size which can be controlled by
adjusting the water content. Solubilization of water in the reverse micelles can be
expressed by w, the ratio of water to surfactant concentrations (w = [H2O]/[surfactant]).
w is an important parameter in determining the size of the reversed micelles and the
structure of water. For a typical spherical AOT reversed micelle, there is a linear
relationship between the diameter of the water pool (D) and w. D = 0.3 w when w is
larger than 15 (Pileni, et al., 1985). In addition, w is related to the structure of water. For
an AOT reverse micelle, when w increases, the structure of the water changes from
bound water to free water.
Due to the controllable water pool, reversed micelles are particularly favorable for the
preparation of monodisperse nanoparticles with various particle sizes. The nanoparticles
can be fabricated using the reversed micelles having the following two features: (1) the
nanoparticles are harder to aggregate because the surface of the nanoparticles is covered
with surfactants; (2) the surface of the particles can be modified further.
Preparation of nanoparticles using reverse micelles can be dated back to the pioneer
work of Boutonnet et al. (Boutonnet, et al., 1982). In 1982 they first synthesized
monodispersed Pt, Rh, Pd, Ir nanoparticles with diameters of 3–6 nm. After that, many
nanoparticles were synthesized and the method of preparing nanoparticles using reverse
micelles became a world wide interest in nanoscience and nanotechnology. In the
following sections we will review the synthesis of various nanoparticles using the
technique of reversed micelles.
The general method to synthesizing nanoparticles using reverse micelles is
schematically illustrated in Fig. 1.8. This can be divided into three cases. The first one is
the mixing of two reverse micelles. Due to the coalescence of the reverse micelles,
exchange of the materials in the water droplets occurs, which causes a reaction between
the cores. Since the diameter of the water droplet is constant, nuclei in the different
water cores can not exchange with each other. As a result, nanoparticles are formed in
the reversed micelles. The second case is that one reactant (A) is solubilized in the
reversed micelles while another reactant (B) is dissolved in water. After mixing the two
9
reverse micelles containing different reactants (A and B), the reaction can take place by
coalescence or aqueous phase exchange between the two reverse micelles.
Figure 1.8 Schematic illustration of various stages in the growth of
nanoparticles in microemulsions (After Leung, at al., 1988).
There are essentially three procedures to form nanoparticles by reversed micelles:
precipitation, reduction and hydrolysis. Precipitation is usually applied in the synthesis
of metal sulfate (Qi, et al., 1996), metal oxide (Ayyub, et al., 1990; 1988), metal
carbonate (Kandori, et al., 1988; Pillai, et al., 1993) and silver halide (Dvolaitzky, et al.,
1983; Hou and Shah, 1988; Chew, et al., 1990) nanoparticles. In this method two reverse
micelles containing the anionic and cationic surfactants are mixed. Because every
reaction takes place in a nanometer-sized water pool, water-insoluble nanoparticles are
formed.
The reduction procedure is one of the most common ways to prepare metal nanoparticles
using W/O microemulsions. By dissolving the metal salts in the reversed micelles, the
salts undergo a dissociation step inside the aqueous domain. Following a reduction step
(Men+ → Me0), a subsequent precipitation of particles can take place inside the water
pools. Strong reduction agents such as N2H4, NaBH4 and sometimes hydrogen gas can
be used.
The hydrolysis procedure is usually used in the preparation of metal oxide nanoparticles.
It utilizes the hydrolysis properties of metal alkoxide dissolved in oil and reacting with
water inside the droplets.
1.3.1 Preparation of Nanoparticles of Metals
Since metals display surface catalytic properties, the synthesis of size-controllable and
monodisperse metal nanoparticles is of considerable importance. The reduction method
is one of the most common ways to prepare metal nanoparticles through reverse micelles.
10
Boutonnet et al. have prepared platinum, palladium, rhodium and iridium nanoparticles
using reverse micelles (Boutonnet, et al., 1982; 1989). H2PtCl6 was dissolved in
CTAB/water/octanol reverse micelle. Subsequent reduction with hydrazine produced
nanoparticles. Pd particles were formed by reducing Pd(NH2)4Cl2 or K2PdCl4 with N2H4.
Rhodium particles were formed by reducing RhCl2 with bubbling hydrogen, whereas
iridium particles could be obtained by bubbling active hydrogen through 2% Pt-Al2O3 at
70°C. Ag and Au colloidial nanoparticles were successfully prepared by reducing the
AgNO3 and HAuCl4 in water/cyclohexane/PEGDE or PEGDE/water/n-hexane reverse
micelles (Barnickel and Wokaun, 1990), where NaBH4 was used as the reduction
reagent. Silver and copper salts of Aerosol OT can be used for the preparation of Ag and
Cu nanoparticles (Lisiecki and Pileni, 1993; Pileni, et al., 1993a; 1993b; Petit, et al.,
1993; Lisiecki and Pileni, 1995). Copper nanosized particles have been synthesized in
the reverse micelles using hydrazine as a reducing reagent. The size of Cu nanoparticles
can be controlled by the water content in the reversed micelles (Lisiecki and Pileni,
1995). Gold and silver nanoparticles were also produced by reducing gold chloride
tetrahydrate HAuCl4 with citric acid at 80°C for half an hour (Chen, et al., 1996; Frens,
1973; Enustun and Turkevich, 1963). Nanoparticles of other metals such as Co (Chen, et
al., 1994; Eastoe, et al., 1996), Ni (Lopez-Quintela and Rivas, 1993) and metal alloys
FeNi (Lopez-Quintela and Rivas, 1993), Cu3Au (Sangregorio, et al., 1996) and Co-Ni
(Nagy, 1989) have also been synthesized using the reversed micelles.
1.3.2 Preparation of Nanoparticles of Metal Sulfides
Colloidal semiconductors are attracting much interest due to their applications as
enhancement of photoreactivity and photocatalysis and non-linear optical properties.
The key to synthetic investigation of this kind of nanoparticles must be the careful
control of semiconductor size and size distribution. The precipitation method is usually
applied in the preparation of metal sulfide particles (Motte, et al., 1992; Hirai, et al.,
1994; Ward, et al., 1993; Boalkye, et al., 1994; Modes and Lianos, 1989). CdS particles
have been synthesized in AOT and Triton reversed micelles with functional surfactant
such as cadmium lauryl sulfate and cadmium AOT (Petit, et al., 1990; Petit and Pileni,
1988). The average diameters of the particles were found to depend on the relative
amount of Cd2+ and S2-. The particles obtained from AOT were smaller and more
monodisperse than those from the Triton reverse micelle. Colloidal CdS was prepared in
the mixed sodium AOT/cadmium AOT/isooctane reverse micelle (Motte, et al., 1992).
PbS nanoparticles can be prepared by mixing one polyoxyethylene dodecyl ether-nhexane reverse micelle, which supplies Pb2+ from electrolytes such as Pb(NO3)2 or
Pb(ClO4)2, and another reverse micelle that contains S2- from Na2S or H2S (Ward, et al.,
1993). A number of nanoparticle semiconductors such as CdS (Lianos and Thomas,
1987; Petit, et al., 1990; Pileni, et al., 1992; Karayigitoglu, et al., 1994), PbS (Ward, et
al., 1993; Eastoe, et al., 1996), CuS (Lianos and Thomas, 1987), Cu2S (Haram, et al.,
1996), Ag2S (Motte, et al., 1996), MoS3 (Boalkye, et al., 1994), CdSe (Steigerwald, et al.,
1988) have also been synthesized using this method.
In recent years apart from the synthesis of nanoparticles, surface modification of the
metal sulfide particles has attracted much interest. The modification of the
semiconductor surface is also very important either from the point of view of enhancing
11
the stability of the nanoparticles or for providing unique physical and chemical
properties. An additional profit from this treatment is that it allows the particles to be
separated from the micellar solution and redispersed in another solvent. Some surfacecapped semiconductor nanoparticles have been synthesized with the cap agents such as
sodium hexamephosphlate (Meyer, et al., 1984; Petit and Pileni, 1988) of the surfacecapping agents such as thiophenol and phenyl (trimethyl) selenium (Steigerwald, et al.,
1988; Herron, et al., 1990; Dance, et al., 1984).
1.3.3 Preparation of Nanoparticles of Metal Salts
Many metal salts such as silver halide, metal sulfate and metal carbonate possess unique
properties. Precipitation methods are usually used to prepare the nanoparticles of these
materials. Silver halide nanoparticles were synthesized by reacting AgNO3 with sodium
halides in Aerosol OT W/O microemulsions (Dvolaitzky, et al., 1983; Hou and Shah,
1988; Chew, et al., 1990).
However, metal carbonate nanoparticles such as BaCO3, CaCO3 and SrCO3 were
prepared by bubbling CO2 through the reversed micelle solutions containing the
corresponding aqueous metal hydroxides. Kandori et al. (1987) used the hexaethylene
glycol dodecyl ether (HEGDE)/water/cyclohexane and calcium AOT based reverse
micellar system to synthesize CaCO3 nanoparticles with diameters of 5.4 nm. The
nanoparticle diameter from the CaAOT system was 48–130 nm (Kandori, et al., 1987;
1988). Nanoparticles of metal sulfate can also be synthesized by the precipitation
method. Nanoparticles of AgCl (Bagwe and Khilar, 1997) and AgBr (Chew, et al., 1990;
Monnoyer, et al., 1996) have been synthesized using reverse micelles.
1.3.4 Preparation of Nanoparticles of Metal Oxides
Nanoparticles of metal oxides are usually produced by the hydrolysis method in which
the metal alkoxides react with water droplets in the reverse micelles. Nanoparticles of
metal oxides such as ZrO2 (Kawai, et al., 1996), TiO2 (Chang, et al., 1994; Joselevich
and Willner, 1994; Chhabra, et al., 1995), SiO2 (Osseo-Asare and Arriagada, 1990;
Wang, et al., 1993; Arriagada and Osseo-Asare, 1995; Gan, et al., 1996; Chang and
Fogler, 1997; Esquena, et al., 1997), GeO2 (Kon-no, 1996), g-Fe2O3 (Lopez-Perez, et al.,
1997) and F2O3 (Liz, et al., 1994) have been synthesized. GeO2 nanoparticles can
directly be obtained from AOT-cyclohexane W/O microemulsions by adding anhydrous
cyclohexane solutions of Ge(OC2H4)4 into the microemulsions. And SiO2 nanoparticles
could be formed by adding Si(OC2H4)4 to the solubilized ammonia aqueous solution in
AOT and polyoxyethylated nonylphenyl ether W/O microemulsions. Similarly, ZrO2
nanoparticles can be obtained by hydrolyzing Zr(OC4H9)4 with sulfuric acid in
polyoxyethylene nonylphenyl ether-cyclohexane systems and then washed with
ammonia aqueous solution. TiO2 nanoparticles can be prepared by adding benzene
solution of TiCl4 to cetylbenzyldimethylammonium chloride-benzene W/O
microemulsions.
12
1.3.5 Preparation of Nanoparticles of Other Compounds
YBa3CuO7-x particles were synthesized by co-precipitation of the oxalate salts of Y, Ba
and Cu nitrates in CTAB/n-butanol/n-octane reversed micelles (Ayyub, et al., 1988;
1990). BaFe12O19 particles were synthesized by the calcination of barium-iron carbonate
particles made by mixing the two reverse micelles containing the (NH4)2CO3 and a
mixture of aqueous Ba(NO3)2 and Fe(NO3)3 (Pillai, V., et al., 1993).
1.3.6 Synthesis of Nanowires Using Reversed Micelles
The nanoparticles fabricated in the reversed micelles are spherical particles in most
cases. However, since the optical, electric, and other properties of nanoparticles are
affected by the shape of nanoparticles, various shapes have been synthesized. For
example, cubic Pt nanoparticles have been synthesized and they showed extremely good
catalysis selectivity and activity (Ahmadi, et al., 1996a; 1996b). Addition of CdS
nanowire into the porous aluminum oxide film will be of potential use in photoelectronics (Routkevitch, et al., 1996). Qi et al., using reversed micelles of TX100/hexanol.cyclohexane/water, have successfully synthesized cubic BaSO4
nanoparticles (Qi, et al., 1997). They have found that the water content in the reversed
micelles greatly affected the shape of the nanoparticles. Cubic nanoparticles of BaSO4
were obtained in the higher content of water. On the other hand, in the non-ionic reverse
micelle C12E4/cyclohexane, adding 0.1 M BaCl2 and Na2CO3 aqueous solution to 0.2 M
C12E4/cyclohexane solution, and mixing the two reversed micelles, BaCO3 nanowires
were obtained (Fig. 1.9). Hopwood and Mann have also synthesized BaSO4 nanowire
using reversed micelles (Hopwood and Mann, 1997).
Figure 1.9 TEM micrographs and electron diffraction pattern of BaCO3
anowires synthesized in reversed micells (Qi, et al., 1997).
13
1.3.7 Synthesis of Composite Nanoparticles Using Reversed Micelles
Composite nanoparticles are composed of two kinds of nanoparticles, not only modifing
the properties of single semiconductor nanoparticles, but also producing some new
electric and optical properties. The composite semiconductor nanoparticles can be
divided into sandwich type and shell-core type. Sandwich type CdS-TiO2 (Spanhel, et al.,
1987; Gopidas, et al., 1990; Lawless, et al., 1995 and CdS-SnO2 (Nasr, et al., 1997) have
been prepared and show prospects in solar cell application. On the other hand, shell-core
type composite nanoparticles such as CdS-ZnS (Hirai, et al., 1994), CdS/PbS (Zhou, et
al., 1993; 1994), CdS/HgS (Hasselbarth, et al., 1993; Mews, et al., 1994; Schooss, et al.,
1994; Kamalov, et al., 1996; Mews, et al., 1996), CdS/Ag2S (Han, et al., 1998),
CdS/CdSe (Tian, et al., 1996; Peng, et al., 1997), CdSe/ZnS (Kortan, et al., 1990; Hines
and Guyot-Sinnest, 1996; Dabbousi, et al., 1997), CdSe/ZnSe (Hoener, et al., 1992;
Danek, et al., 1996) have been synthesized using different methods. They showed
enhancement of photocatalytic efficiency and strong enhancement of emission.
Reversed micelle is also an important method for synthesizing the composite
nanoparticles. So far reversed micelles have been successfully used to synthesize
composite nanoparticles such as CdS-ZnS (Hirai, et al., 1994), CdS-Ag2S (Han, et al.,
1998) and CdSe-ZnS (Kortan, et al., 1990), CdSe-ZnSe (Hoener, et al., 1992).
For shell-core type nanoparticles the synthesis contains two steps: the first step is the
formation of core nanoparticles in the reverse micelles and the second step is the growth
of the shell particles on the core. CdS/ZnS (where CdS is the core and the ZnS is the
shell) is a typical shell-core type composite nanoparticles and can be synthesized as
follows. Mixing the reverse micelles containing Cd2+ and S2- in a 1 : 2 ratio, one can
obtain the core CdS reverse micelle solution. In this reversed micelle S2- is excess. After
several minutes, Zn2+ containing reverse micelle was added. ZnS precipitated in the core
CdS nanoparticles, and a shell-core type CdS/ZnS composite nanoparticle was obtained.
For the ZnS/CdS composite, the same method can be used, only changing the order of
synthesis. Using this method, Ma et al. have prepared composite nanoparticles of
CdS/ZnS and ZnS/CdS. Another type of composite nanoparticle contains two metals not
in the 1 : 1 ratio. They can be synthesized as follows. In the synthesis of coated
Ag2S/CdS nanoparticles, after mixing the two reverse micelles containing the equal
molar Cd2+ and S2-, AgNO3 was added to the mixed reversed micelles. The reaction of
2Ag+ + CdS(s) → Cd2+ + Ag2S. Coated Ag2S/CdS small particles with a diameter of ~10
nm was obtained. The nanoparticles showed large nonlinear absorption.
1.4 Synthesis
Microemulsions
of
Organic
Nanoparticles
from
O/W
1.4.1 Introduction
Only a limited number of organic nanoparticles can be prepared using oil-in-water (O/W)
microemulsions (Gan, et al., 1983a; Atik and Thomas, 1981; Leong, et al., 1984; Candau,
1990), usually called microemulsion polymerization. Stoffer and Bone (1980a, 1980b)
14
first reported using O/W microemulsion in polymerization of methylacrylate and
methylmethacrylate, but found phase separation occurred during the polymerization.
Similar problems have been encountered by Gan et al. (Gan, et al., 1983a, 1983b; Gan
and Chew, 1983, 1984, 1985; Chew, et al., 1989) and Jayakrishnan (Jayakrishnan and
Shah, 1984). Phase separation is perhaps the main reason why such techniques made
little progress within the past decade (Holdcroft, et al., 1990).
Nanosize polymer particles can be obtained using polymerization reactions in O/W
microemulsions (Stoffer and Bone, 1980a, 1980b; Antonietti, et al., 1991). This leads to
hydrophobic polymer nanoparticles (10–40 nm) dispersed in water. The advantages of
this method are fast polymerization rates and high molar masses of polymers, while the
drawback is the need of high weight ratio of surfactant to monomer.
Carver (Carver, et al., 1989) studied the polymerization process using electron
microscopy and found that each polymer particle is formed in a single nucleation step,
the number of particles growing steadily during the polymerization. Figure 1.10
illustrates the polymerization mechanism in O/W microemulsions (Candau, 1990).
Figure 1.10 Schematic representation of synthesis of organic nanoparticles in
O/W microemulsions: I. Before polymerization. II. Polymer particle growth (a)
by collisions between particles, (b) by monomer diffusion through the oil phase.
III. End of polymerization (Candau, F., 1990).
Matching between oils and emulsifiers is the key to making stable latices. The particle
size depends on the nature and concentration of surfactant; usually the lower the
surfactant content, the bigger the size. The presence of electrolyte may help the
formation of stable microlatices (Holtzscherer and Candau, 1988).
1.4.2 Synthesis of Styrene Latex Nanoparticles from O/W Microemulsions
The first successful microemulsion polymerization was reported by Atik and Thomas
(1981), who used CTAB/styrene/hexanal/water O/W micro-emulsion. The reaction was
carried out either thermally using azobisisobutyronitrile (AIBN) or radiolytically using
Cs γ-ray source. Monodisperse latex nanoparticles of diameters 35 and 20 nm were
obtained, respectively.
Styrene has also been polymerized using three component microemulsions of dodecyl
trimethyl ammonium bromide (DTAB) and potassium persulphate (KPS) initiator
15
(Perez-Luma, et al., 1990). This resulted in monodisperse latices with radii in the range
of 20–30 nm. Guo et al. (1989) studied styrene polymerization in SDS/pentanol/water
microemulsions using both water soluble KPS and oil soluble AMBN as initiators, and
found that the fraction of formed particles was determined by the amount of initiator.
Kuo, et al. (1987) studied photo initiated polymerization of styrene in O/W
microemulsions using dibenzyl ketone as initiator. Uniform nanosize latices were
formed. Styrene may also be polymerized in anionic (AOT) and non-ionic (Neodol 91-5
and Emsorb) microemulsions (Qutubuddin, et al., 1989). In the ionic system, gelation
took place during polymerization; while in the non-ionic system the microemulsion first
became a gel after which polymerization started. Temperature control is usually
important for nucleation and growth of the particles.
Co-polymerized styrene with divinylbenzene (DVB) in microemulsions of CTAB and
hexanol was studied by Atik and Thomas (1982). The size of spherical particles was
between 20 and 40 nm. The particles were charged, had a distinctly rigid core, and were
very stable when the latices were diluted.
1.4.3 Synthesis of Methylmethacrylate Nanoparticles from O/W Microemulsions
Styrene and methylmethacrylate (MMA) microemulsion polymerization was
investigated by Jayakrishnan (Jayakrishnan and Shah, 1984). Non-ionic surfactant
(Pluronic L-31) and the oilsoluble initiator AIBN and benzoyl peroxide (BP) were used.
However, the microemulsion systems did not remain stable during the polymerization
process. As a result, nonspherical particles with low stability were formed.
Palani Raj, et al. (1991) studied the polymerization of MMA using MMA/ethylene
glycol dimethacrylate/water systems with acylamide as amphiphile. The particles
formed were transparent up to 60% of water in the microemulsion systems.
1.5 Applications
Since the particle size of nanoparticles is in the order of nanometers (1–100 nm), both
electron distributions and atom positions at the surface may be different from those at
the bulk. Thus, nanoparticles show many outstanding characteristics that the bulk
materials do not possess. The most obvious properties are surface effect, quantum effect
(Kubo, 1962; Wang and Herron, 1991), mini-size effect and macroquantum channel
effect (Legget and Chakravarty, 1987; Awschalom and McCord, 1990). It is these
special properties that make nanoparticles attractive to many researchers, and
nanoparticles have found many novel applications in the electronic, metallurgical,
chemical, biological and pharmaceutical industries.
There are two important applications for nanoparticles prepared through microemulsion
routes. One application is the synthesis of high performance materials, such as
superconductivity materials, smart materials, coating materials for chemical or
biological sensors, etc. Another application is drug delivery systems, which may have
potential market in biomedical industries.
16
1.6 Prospects
It is no dispute that nanoparticles will play an important role in the future advanced
materials and in medicine and biology. For these applications some aspects should be
addressed and could be improved. First, one needs to develop the synthesis methodology
of the nanoparticles with designed function and properties. Second one must solve the
problem of the long-term stability of nanoparticles in their applications. Finally one
should establish the strategy of assembling ordered nanoparticles in multifunctional
devices. Biocompatibility is also a key element concerning the application of
nanoparticles in biological systems.
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2.1 Introduction
Microelectronic technologies have brought a profound revolution in human lives over
the past years with continuing developments on the way. As an extension
nanotechnology has appeared in the lab in the 1980's, a lot of new phenomena have been
observed since then. These phenomena show contrasting characteristics as compared
with that of micro-technologies. Right now although it is difficult to imagine clearly
what will happen in the future, nanotechnology is predicted to change our lives more
drastically than micro-technologies. "Nanotechnology" is a new interdisciplinary field
based on the materials and devices on a nanometer regime. The basis of nanotechnology
is the condensed matter with quantized behavior, collective effect and a highly
assembled nano-unit with new functions. In spite of the nanotechnology's exciting and
promising perspective, the nanoscience should go first to understand the whole picture
of the nanotechnology. As branches of nanoscience, nanochemistry and nanophysics
studies show many interesting phenomena at the atomic or molecular scale, which
induce many new branches of nanoscience, such as nanomaterial science,
nanoelectronics, nanophotonics, nanomechanics, nanomagnetics, and so on. Most of the
new developments in these nanosystems will be covered in this book.
From the point of material preparation, the difference between nanophysics and
nanochemistry lies in that for nanophysics one makes nanosystem from the bulk
substance and for nanochemistry people prepare nanomaterials from molecules or ions.
The properties of these nanosytems are different from the bulk materials and molecules
or ions. So, "nanomaterial research", as an important branch of nanoscience, has gone
ahead fastest as a combination and complement of nanochemistry and nanophysics.
Many nanomaterial systems with specific structure and properties are synthesized
(Alivisatos, 1996), for example, the self-assembled metal particles (Harfenist, et al.,
1996), insulator (Yin, et al., 1997) or semiconductor nanocrystal arrays(Motte, et al.,
1996), the intrataxy growth of nanoparticles in zeolites(Agger, et al., 1998),
nanoparticles array in 2-D Langmuir-Blodgett films (Peng, et al., 1992).
The first step of nanomaterial research is the preparation of uniform nanoparticles and/or
nanoparticle arrays with correct chemical composition and structure. There are many
chemical methods used, such as coprecipitation, reverse micelle, vesicles hydrothermal,
pyrolysis, ion sputtering, electrochemical, CVD (chemical vapour deposition),
sonochemical dissociation, forced hydrolysis, sol-gel, guest growth in host, metalorganic reaction etc. As a sidebranch of nanomaterialpreparation, both pyrolysis and
23
hydrolysis techniques are the oftenest used methods and played greater roles in the
chemical preparation of various nanomaterials. Because hydrolysis methods are
discussed in Chapter 3 and 4, this chapter will focus on the pyrolysis technique.
Pyrolysis is a chemical process in which chemical precursors decompose under suitable
thermal treatment into one solid compound and unwanted waste evaporates away. Upon
completion the wanted new substance is obtained. For example, the reaction
produces the important construction material Ca(OH)2. This phenomenon was been used
much earlier in ancient China. In fact many other materials have been prepared with this
traditional pyrolysis technique.
Generally, the pyrolytic synthesis of compounds leads to powders with a wide size
distribution in the micrometer regime. To get a uniform nanosized material some
modifications or revisions of the pyrolytic preparation procedure and reaction conditions
are added. There are many revisions used to make nanomaterials by pyrolysis, for
example: (1) atomize the precursor solution; (2) use stable matrix, i.e., zeolite molecular
sieves or glass, to disperse the precursor solution; (3) slow down the reaction rate to
obtain nanoparticle film, i.e., the oxide superconducting films formed in a vacuum; (4)
allow the reaction occur in the inert solvent or inert gas; (5) use a decomposable
polymer or molecules to disperse and protect precursors and as prepared nanoparticles.
These revisions effectively decrease the critical formation temperature of nanoparticles
and protect the nanoparticles from aggregation and agglomeration. These methods
usually can be combined to synthesize specific materials for different applications.
1. Generally used precursors According to the above reaction principle, general used
precursors are MCO3 (M-metal ion), MC2O4, M (C2O2)2, M(CO)x, MNO3, glycolate,
citrate, alkoxides, MOCVD (metal organic chemical vapor deposition) used
organometallic compounds, some metal ion complex or chelate. The additives such as
polyvinyl alcohol (PVA), polyethylene glycol(PEG), etc. can be used to be the
protecting agents. The reaction generally occurs as follows: (M—metal ion or metal
element)
2. Generally used technique This method uses various thermal resources such as
furnaces, lasers, ultrasonic equipment, electric discharge, microwave, plasma, as well as
many others, to heat up the precursors or increase the local temperature. These thermal
24
resources can supply uniform or transient thermal energy to decompose the precursor
compounds. Some of these thermal resources can even supply energy in the picosecond
or nanosecond timescale within a very small space, which facilitate the control of the
reactants to products more accurately to obtain advanced nanomaterials. The precursors
may be chosen on the basis of their easy availability, ease of decomposition and the
volatility of decomposition by-product. The decomposition temperature of the
precursors and ambient environment should be taking into consideration for the
preparation of a specific system.
A typical example of pyrolysis, the preparation of Fe nanoparticles incoporated in
silicate film, is reported in Li, et al. (1996). At first the Fe(NO3)3 aqueous solution was
mixed with the sol formed from TEOS (tetraethyl orthosilicate). Then the sol was used
to make a film by spin-coating. After the film is dried by sintering in a tube furnace at
around 500°C for 1 hr in air, is passed H2 through it. In the end the Fe nanoparticles
incorporated in the SiO2 film were obtained. Fe nanoparticles on the surface of this film
can catalyze the chemical growth of arrayed carbon nanotube, which open a new way to
the future applications of carbon nanotube arrays.
3. Sections in this chapter Because many substances have been successfully prepared by
the pyrolysis method until now, and the dispersion condition and reaction processes of
these nanomaterials varied from substance to substance, it is hard to discuss them in
detail within one chapter. Therefore we categorize these works into several sections
from the type and properties of the materials. The first section is the preparation and
characterization of metal nanoparticles, and some specific properties are indicated. The
second section is the preparation and characterization of oxide nanoparticles, which are
divided into the ceramic oxide nanosystems and the functional oxide nanomaterials. The
third section discusses the pyrolytic synthesis of compound semiconductor nanoparticles
and related properties. The fourth section concerns the preparation and characterization
of high-Tc superconducting cuprate nanosystems and their strongly reaction processdependent properties. The fifth section is about the element nanosystems, which include
the famous carbon nanosystems like fullerene, and luminescent IV semiconductor
nanoparticles. The sixth section introduces a little bit of new techniques for
nanoparticles in mesoporous materials—intrazeolite topotaxy. In the end the seventh
section gives the overview on the pyrolytic methods for new nanomaterials. The
presentation of the preparation methods mainly focuses on the specific examples by
different groups in the world.
2.2 Metal Nanoparticles
2.2.1 Background
Metal nanoparticles have attracted much attention for a long time due to their theoretical
and application importance in many industry as well as high-tech fields. Quantum
confinement, the size effect contributed by the quantum mechanics, plays an important
role in the optical, electrical and magnetic properties of nanosystems (Halperin, 1986).
25
Many new properties in contrasting to that in traditional bulk metal have been observed.
In fact these properties also strongly depend upon the composition, structure,
aggregation, environment as well as the matrix where the nanoparticles stay. There is a
strong relation to the preparation process. Recent study on the single electron effect
(Ahmed, 1997) and metal nanoparticle arrays (Harfenist, et al., 1996) gives the similar
conclusions. The newly discovered physical properties in these nanosystems show a new
direction for the future microelectronic applicaions.
2.2.2 Precious Metal Nanoparticles
The simplest example of pyrolysis synthesis is the preparation of Ag nanoparticles.
AgOH precipitate forms in AgNO3 aqueous solution after adding basic solution. Then
heating the precipitate for a time will yield Ag powder.
Many precious metal nanoparticles can be obtained through above procedure yielding a
wide size distribution from 10nm to 1000nm, depending upon the reaction conditions
and reaction media. First-periodical transition metal nanosystems are unstable in air, and
need to be obtained within the inert gas because they are easily oxidized by oxygen.
Incorporating the precursors into the stable porous matrix, 1–100 nm nanoparticles can
be obtained. These nanoparticles are very important to the catalysis and information
storage applications. Cai et al. (Cai, et al., 1997) recently incorporated Ag nanoparticles
into the mesoporous silica using sintering of the silver salt with MCM-41, and as
prepared samples show semiconducting properties. However this technique may not
remove the possibility that particles are adsorbed outside the pore of MCM-41.
Au nanoparticles can also be obtained with a similar method. Maya, et al. (Maya, et al.,
1996) prepared gold oxide films by reactive sputtering of pure gold in oxygen plasma.
Gold oxide, Au2O3, decomposes into the elements at 350°C. It does not react with dry
carbon dioxide but does form a metastable bicarbonate in the presence of moisture and
CO2, releasing oxygen and eventually reverting to elemental gold nanostructures. Gold
oxide could also be generated by reactive sputtering along with silica in oxygen plasma
from Au-Si solidified alloys. Gold oxide decomposed upon pyrolysis to produce
composites showing different characteristics depending on the gold content. Composites
containing about 95 wt% gold produced reflective, conductive, and adherent films.
Composites derived from an alloy containing 5 wt% gold produced a nanostructured
material with gold clusters of about 5 nm in diameter dispersed in a silica matrix. This
nanocomposite showed high resistivity, and capacitance with a dielectric constant of 400.
These results actively reflected the variety of pyrolysis applications and as prepared
materials.
In order to develop new materials with nonlinear optical properties, Vacassy et al.
(Vacassy, et al., 1998) reported the sol-gel synthesis of zirconia (ZrO2) gel-coated gold
nanoparticles and their characterization using TEM (tunneling electron microscopy),
FTIR (Fourier transform infrared spectrometer) and UV (ultraviolet)-visible spectra.
26
Because ZrO2 has different dielectric constant and properties from Au, the above design
actually supplied a system to study the quantum confinement and dielectric confinement
effects on the optical properties of metal nanoparticles.
Silver nanoparticles of high chemical homogeneity have been synthesized by a novel
laser-liquid-solid interaction technique (Subramanian, et al., 1998) from a solution
composed of silver nitrate, distilled water, ethylene glycol and diethylene glycol.
Ethylene glycol and diethylene glycol are used to protect the nanoparticles from growth.
Rotating nickel, niobium, stainless steel, and ceramic Al2O3 substrates were irradiated
using a continuous-wave CO2 laser and Q-switched Nd: YAG laser (λ = 1064 and 532
nm). The silver nanoparticles were characterized using X-ray diffraction (XRD),
scanning electron microscopy (SEM), and electron probe X-ray microanalysis (EPMA).
The shape of silver particles was dependent on the chemical composition and laser
parameters. The synthesis mechanism of silver nanoparticles has been proposed to occur
primarily at the laser-liquid-substrate interface by a nucleation and growth mechanism.
2.2.3 Transition Metal Nanoparticles
Typical first row transition metal nanoparticles cannot be obtained with the above
method. However, they can be obtained by modifying the preparation procedure, for
example, reducing their oxide nanoparticle with H2 or another reductant gas at a suitable
temperature. The reactions
occurred in the porous silica film (Li, et al., 1996) can form nanoparticles in SiO2 films;
and reactions
could occur on the silicon wafer or glass substrate by atomization and pyrolysis. Nickel
nanoparticles were formed after hydrogen reduction at 500°C in a tube furnace. Figure
2.1 is the SEM (Scan Electron Microscopy) image of Ni nanoparticles film on the
surface of silicon wafer. These nanoparticles have proved to be good catalysts for the
carbon nanotube array growth.
Figure 2.1 Ni nanoparticles on the surface of silicon wafer made by Ni (NO3)2
decomposition.
27
M (CO)x is another typical precursor for the preparation of metal nanoparticles. CO or
CO2 lasers are often used to decompose these precursors because the carbonyl group can
strongly absorb the light emitted from the laser (Mingfei Zhou, et al., 1998). Fe-C alloy
nanomaterials also (Xiang-Xin Bi, et al., 1993) can be generated with the laser pyrolysis
in addition to Fe. Recently ultrasound has been proved extremely useful in the synthesis
of a wide range of nanostructured materials, including high surface area transition
metals, alloys, carbides, oxides and colloids (Suslick, 1998). The sonochemical
decomposition of volatile organometallic precursors in high boiling-point solvents has
been used to produce nanostructured materials of various forms with high catalytic
activities. Figure 2.2 shows a typical laboratory apparatus for carrying out sonochemical
reactions. This route can prepare all nanometer colloids, nanoporous high surface-area
aggregates, and nanostructured oxide supported catalysts. For example, the recent
development of a simple sonochemical synthesis of amorphous iron nanoparticles (Fig
2.3) helped settle the longstanding controversy over their anomalous magnetic properties.
Figure 2.2 Typical laboratory apparatus for carrying out sonochemistry.
28
Figure 2.3 Scanning electron micrograph of amorphous nanostructured iron
powder produced from the ultrasonic irradiation of Fe (CO)5. Particles making
up this porous agglomerate are 10 to 20 nm in diameter. (from Depero et al.,
1994)
The easiest way to use an ultrasound source is to decompose metal carbonyl to produce
nanoparticles. Before the reaction the reactants are dissolved in the organic solvent or
organic media to protect the particles from being oxidized (Jingu Lin, et al., 1995;
Suslick, et al., 1991; 1996). Fe-Co (Wang, et al., 1996) alloy nanoparticles were also
prepared in the same way. The sonochemical technique is also used to make other kind
of nanoparticles using different precursors (Hyeon, et al., 1996). These indicated that
this method has great potential in the preparation chemistry.
Peigney et al. (Peigney, et al., 1998) reported the production of carbon nanotubes-Fealumina nanocomposites. In the experiments, oxides of α-alumina and containing
varying amounts of Fe (2 wt%, 5 wt%, 10 wt%, 15 wt% and 20 wt%) were prepared by
decomposition and calcination of the corresponding mixed oxalates. Selective reduction
of the oxides in an H2-CH4 atmosphere produces nanometer Fe particles, which are
active for the in situ nucleation and growth of carbon nanotubes. These nanotubes form
bundles of smaller than 100 nm in diameter and several tens of micrometers long.
29
However, the carbon nanotubes-Fe-Al2O3 nanocomposite powders may also contain Fe
carbide nanoparticles as well as undesirable thick, short carbon tubes and thick graphene
layers covering the Fe/Fe carbide nanoparticles. The influence of the Fe content and the
reduction temperature on the composition and micro/nanostructure of the nanocomposite
powders have been investigated with the aim of improving both the quantity of
nanotubes and the quality of carbon, i.e. a smaller average tube diameter and/or more
carbon in tubular form. A higher quantity of carbon nanotubes is obtained using αAl1.8
Fe0.2O3 as starting compound, i.e. the maximum Fe concentration (10 wt%) allowing
retention of the monophase solid solution. A further increase in Fe content provokes a
phase partitioning and the formation of a Fe2O3-rich phase, which upon reduction
produces too large Fe particles. The best carbon quality is obtained with only 5 wt% Fe
(αAl1.9Fe0.1O3), probably because the surface Fe nanoparticles formed upon reduction are
a bit smaller than those formed from α-Al1.8Fe0.2O3, thereby allowing the formation of
carbon nanotubes of a smaller diameter. This is a typical example of how the chemical
compositions of as-prepared nanosystems determine their properties except the sizes of
the particles.
Dense spherical Ni particles were prepared by Che et al. (Che, et al., 1999) from nitrate
solution by spray pyrolysis in an H2-N2 atmosphere. Hollow NiO particles with rough
surfaces were generated first at low temperature and then reduced to Ni by H2 above
300°C. Subsequent intraparticle sintering of the Ni crystallites gave rise to densification
of Ni particles as the temperature was raised, and most Ni particles became dense above
the pyrolysis temperature of 1000°C. However, when a N2 atmosphere was used, hollow
NiO particles were formed, which did not densify even at 1200°C due to the lack of
sintering. The dense Ni particles obtained were of good crystalline and good oxidation
resistance, especially for those formed at higher pyrolysis temperatures and longer
residence times. Ni nanoparticles can also be inserted into zirconium phosphate layer
(Ayyappan, et al., 1996) together with NiO nanosystem by the thermal decomposition of
nickel acetate intercalated and characterized by EXAFS and magnetic measurements.
The nickel nanoparticles are superparamagnetic. Hydrogen reduction produces small
ferromagnetic nickel particles: however, most of these particles appear to be outside the
interlayer space of ZrPO3.
Based upon the above discussions it is concluded that the pyrolysis technique is
powerful for the preparation of various metal nanosystems with varying composition,
shape, structure and properties. According to the applications and properties, different
matrix or stabilizing agents can be used to protect the nanoparticle from growth.
2.3 Oxide Nanoparticles
2.3.1 General Background of Nano-Oxides
Oxides are the commonest-seen minerals in the earth, and the number and the variety of
present oxides are not comparable by any other type of compounds. Not all of them are
of significance for the laboratory and industry preparation. Oxide nanoparticles have
30
wide applications, from ceramics, catalysis, sensor, to electronics, optics and magnetics,
especially the transition metal oxide (TMO). These applications mainly originate from
their rich valence states, vast surfaces, and varying electronic structures. For example,
transition metal oxide can show insulator to conductor due to the chemical stoichiometry
or the variation of valence state. Some oxide compounds even have superconducting and
giant magnetic resistance properties after adjusting the composition. Some of them may
show photo- or electric-chromic properties. Some of them are ferromagnetic or
antiferromagnetic or paramagnetic, and some of them have ferroelectric properties. The
variety of their magnetic, transport and optical properties has attracted interest for a long
time: for clarity, this chaper can be divided into two parts based upon the properties and
applicable functions of these oxide nanoparticles.
Transition metal oxide (TMO) nanoparticles can be easily prepared with pyrolysis: for
example, the reactions (2.2), (2.6), (2.8). The key point of this synthesis technique is that
before heating the precursors one need to atomize the solution or incorporate into pores
in the porous matrix, or mixed with dissolvable polymers, so that the nanoparticles as
prepared will not grow during suitable sintering. That is, the most important step is to
protect nanoparticle as formed from aggregation and agglomeration. After sintering,
some of the well-separated oxide nanoparticles can be used for catalyst together with the
matrix or substrate. However, free-standing nanoparticles are also important for many
applications, such as paint, drug, or ink, etc. In fact most important oxide nanosystems
are composite oxides, which generally have properties adjustable to chemical condition,
perovskite, one type of TMO, have a variety of composition and properties. It is obvious
that the above pyrolytic technique also works for the composite oxides if suitable
precursors and mixing techniques for them are available. Some successful examples
have been reported in recent publications. However, those reactions often need higher
temperature condition and complicated pre-processing or pre-mixing. Higher
temperature will lead to larger particles and aggregation of particles, which sometimes
cause nonstoichiometry and a lot of defects in the nanoparticles. Bad mixing may lead to
impurity formation in the nanoparticles. So the pyrolysis technique may be not as
preferable as sol-gel processing for composite TMO oxide nanoparticles.
2.3.2 Ceramic Oxide Nanoparticles
The first type of these oxide nanomaterials have very specific mechanical properties and
stability as structural materials, for example Al2O3, TiO2, ZrO2, as well as their
composite oxide and silicates. They are very important in the ceramic, paint, ink and
many other industry fields. Their stable, monodispersed and unaggregated nanoparticles
will facilitate the formation of high quality ceramics or glasses. So many methods have
been used to prepare the nanoparticles of these oxides; some of them already have been
used for industry production.
A novel method, which involved the heating of ZrOCl2 and Y (NO3)3 solution with an
alcohol-water mixture as a solvent, was used to synthesize ZrO2/(3Y) nanoparticles by
Li et al. (Li, et al., 1998). By choosing the ratios of alcohol to water and adding
appropriate dispersant, weakly agglomerated ZrO2(3Y) powder with particle size of
about 11–15 nm could be obtained. Contrast with the ordinary trend of the powder
31
synthesized by coprecipitation, the monoclinic phase in this powder decreased when the
calcinating temperature increased from 600°C to 900°C; the mechanism of this
phenomenon was investigated. Preliminary compaction and sintering studies indicated
good compactability and sinterability of the powder. Measures need to be taken to avoid
chloride impurity in the product of this method.
Sometimes composite oxide may show advantages of individual oxides. Al2O3-TiO2
composite oxide nanocrystals with different Al2O3-TiO2 ratios (1/4, 2/3, 3/2 and 4/1)
were prepared in Xu's group (Gang Xiong, et al., 1998) by the poly(ethylene glycol)
(PEG) sol-gel method. The preparation process was monitored by thermogravimetric
analysis and differential scanning calorimetry (TGA-DSC). Nanocrystal particles of
these composite oxides obtained at various heat-treating temperatures (400–1100°C)
were characterized in terms of morphology, size, specific surface area, composition and
structure by transmission electron microscopy (TEM), BET specific surface area
analysis, and X-ray powder diffractometry (XRD). Nanoparticles of Al2O3-TiO2 with
grain sizes in the range 1–150 nm and specific surface areas of 4.3–136 m2/g could be
obtained under different conditions. The morphology of the particles changed from
spherical to cubic with increasing heat-treatment temperature. Anatase was stabilized in
these composite nanomaterials and the mechanism was discussed. The change of the
particles specific surface areas with increase of the Al2O3-TiO2 ratio was investigated.
The Al2O3-TiO2 composite oxide nanocrystals could catalyze the polymerization of
maleic anhydride and may provide a route to obtain a product without rings at the end
groups. This method has been widely used for various oxide nanosystems with low cost
and variable heat treatments.
Indackers et al. (Indackers, et al., 1998) reported the synthesis of Al2O3 and SnO2
particles by oxidation of organometallic precursors in premixed H2/O2/Ar low-pressure
flames. In their experiments, low pressure premixed H2/O2/Ar flames were doped with
the metalorganic precursors Al (CH3)3 (trimethylaluminum) and Sn(CH3)4
(tetramethyltin), respectively. The dopant concentration in the feed gas mixture was
varied between 96 ppm and 3066 ppm. Nanosized Al2O3-particles and SnO2-particles
were formed during the oxidation process. They were extracted at different heights from
the flame zone by two thermophoretic sampling devices and a molecular beam probe,
which is part of an aerosol mass spectrometer (AMS). This instrument allows the in situ
analysis of the combustion aerosol according to the chemical composition of the gas
phase as well as the mass of charged particles. The thermophoretically sampled particles
were analyzed for their chemical composition, specific surface area, crystal structure,
particle size, and morphology by the use of FTIR absorption spectroscopy, BET gas
adsorption method, X-ray/electron diffraction, and bright-field TEM analysis. The gas
phase of the combustion aerosol was analysed for gaseous reaction products of the
precursors. Neither in the Al(CH3)3 doped flames nor in the Sn(CH3)4 experiments are
gaseous metals or metal compounds found. Measuring the formation of the by-product
CO2 monitored the oxidation of Al(CH3)3. At x = 35 mm (reaction chamber pressure) the
slope of the CO2 concentration profile indicates complete oxidation of the precursor. The
formation and growth of amorphous Al2O3 particles of spherical shape were about 4.7
nm. However, the drawback of this reaction is the expensive cost of the precursors.
32
Using laser-induced pyrolysis, powder samples of pure TiO2 and mixed oxides with
different vanadium content were prepared and analyzed by XRD in Casale's group
(Depero, et al., 1994). The diffraction patterns were interpreted in the microstructural
terms by Fourier analysis of their peak profiles. The influence of vanadium on the phase
transition was studied and the changes in the particle and microstrain distributions
obtained at different temperatures were analyzed. There is an evident correlation
between the initial microstrain distribution in the Ti1-x VxO2 powder and the vanadium
content. The segregation of the V2O5 phase causes a strain reduction into the anatase
structure. It is suggested that the previously observed lowering of the transition
temperature for the anatase-to-rutile transformation in the presence of vanadium is due
to distortions induced by this ion in the anatase structure. Experimental results prove that
laser-induced pyrolysis is an excellent method for specific composite oxide nanosystems.
2.3.3 Specific Ceramic—SiC
There is another excellent ceramic material, non-oxide compound—SiC, for the use in
the metallurgical industry and as an abrasive material. Generally SiC powder is
synthesized by sintering Si and C at temperature higher than 2000°C. Although many
alternative methods have been exploited, these methods using various chemical
precursors to prepare SiC are also expensive. Many researchers have been trying a cheap
route to synthesize SiC nanoparticles. Riedel and Gabriel (Riedel, et al., 1999) recently
proposed an economic method using a liquid-phase process involving the thermal
conversion of poly(methylsilasesquicarbodiimide) into nanosized silicon carbonitride at
700°C, followed by its final crystallization at >1400°C. This is an exciting advance.
Interestingly Fu et al. (Zhengping Fu, et al., 1998) even proposed another pyrolytic
method of decomposing the Langmuir-Blodgett films of polyimide containing dispersed
silicon nanoparticles to get SiC nanoparticle films. CVD technique (Huang, et al., 1996)
is also proved to be a good method to obtain SiC nanoparticle powders. It can be
predicted that better methods for SiC nanoparticles will be devised in the future due to
their importance in the industry and human life.
Limited by the volume, many other ways to obtain ceramic nanomaterials (Nedeljkovic,
et al., 1997; Inoue, et al., 1996; Blanchard, et al., 1994; Skandan, et al., 1998; Skandan,
et al., 1997) can not be discussed in detail here. Interested readers can refers to the
specific review articles and books on ceramics.
2.3.4 Functional Oxide Nanoparticles
Another type of oxide nanoparticle is more widely studied due to its characteristic
physical properties and potential applications, which include electro-optic, luminescent,
magneto-optic, sono-optic, ferroelectric, piezoelectric, electromagnetic absorption,
photoelectric, photo or electrochromic properties. These properties are the basis of
modern electronics. TMO, such as titanates, niobates, manganite are the typical
substances showing the above physical properties. Some cuprates—high-temperature
superconductors are also examples of functional materials, which will be discussed later.
33
Yang et al. (Yang, et al., 1997) have successfully prepared magnetoresistive La-Sr-MnO powders and films by deposition of aqueous acetate solution (DAAS) technique. This
novel technique, which has the potential for depositing large area thin films with high
throughput and low cost, involves the preparation of an aqueous metal acetate precursor
solution, drying the solution to generate a glassy gel, consolidating the gel, and then
firing it for short periods of time (<2h) to produce crystalline lanthanum strontium
manganate (LSMO). The DAAS method has been used to prepare both powders and thin
films of LSMO. Powder samples of La0.83 Sr0.17 MnO3 annealed for 100 min at 1200°C
were high purity, single phase, and exhibited excellent electrical and magnetic
characteristics. Thin films of La0.7Sr0.3MnO3 deposited onto both sapphire and strontium
titanate substrates and annealed at 900°C were also found to be crystalline, and substrate
choice was found to influence thin film crystal structure. These films exhibited sharp
metal-insulator transitions and in the case of LSMO films on strontium titanate,
magnetoresistance was observed at the unusually high temperature of 360 K. Du et al.
(Wei, et al., 1997) have reported the preparation and magnetic properties of barium
hexaferrite nanoparticles produced by the citrate process together with the sol-gel
technique. Several methods such as X-ray diffractometry, TEM and magnetism
measurement have been used to obtain detailed information on the crystallography and
magnetic properties of the precursor and the calcined particles. The correlation
BaFe12O19 formation with the precursor properties and thermal treatment has been
studied. The optimum conditions for preparing BaFe12O19 nanoparticles at a low
calcining temperature are reported and homogeneous ultrafine BaFe12O19 with almostideal single-domain behavior and coercive force (5950 Oe) and specific magnetization
(70 emu/g) values that are similar to the theoretically predicted values are obtained.
Sato et al. (Sato, et al., 1998) reported the electrochromic properties of spin-coated
nickel oxide films. In their work, nickel 2-ethylhexanoate was spin-coated from organic
solution, followed by pyrolysis in air at 380°C. The scanning electron microscopy (SEM)
and X-ray diffraction (XRD) analyses showed that the spin-coated films consisted of
fine particles of NiO crystallites, 10–20 nm in diameter. The optical response and
cycling ability of the films were measured by applying a rectangular potential pulse
between 0.0 and 0.5 V vs. SCE in a 1 M KOH electrolyte. The influence of the
processing temperature on the electrochromic properties of the films was studied.
Although the films pyrolyzed in air at 380°C showed the response time of 1–2 s for
coloration/bleaching, the degradation was observed within a few thousands to 104 cycles.
The cycling ability could be much improved by post heattreatment at 500°C for 20 min
in air without sacrificing the response speed. This technique works very well for the
preparation of nanoparticle film.
Tissue and Eilers (Tissue, et al., 1996) reported the preparation and characterization of
nanocrystalline particles of ZnO, Eu2O3, and Eu3+ : Y2O3 by CW CO2 laser vaporization
and gas-phase condensation of metal oxide ceramics. Particle diameters range from 5–
280 nm for ZnO and 2–30 nm for Eu2O3, for a variety of preparation conditions. ZnO
forms in the usual room-temperature hexagonal crystal structure but Eu2O3 and Y2O3
crystallize in a metastable monoclinic phase. The optical spectra of 14 nm Eu2O3
particles have the same sharp lines as furnace-heated monoclinic Eu2O3. The spectra of
5nm Eu2O3 particles show the sharp lines superimposed on broad and shifted bands. The
34
spectra of 12